Sombrero and the Ultra-Compact Dwarfs

This week’s OOTW features Jean Tate’s OOTD posted on the 28th of September.

This is M104, otherwise known as the Sombrero galaxy. Lurking in the picture above hanging in front of the galaxy’s halo and blending in with all the stars of our galaxy is a strange little thing; an Ultra-Compact Dwarf (UCD) called SUCD1 :

UCDs are very compact objects, with millions of stars crowded into a small area as small as 200 light years across! They are rather luminous, showing up on the SDSS as star-like points. The UCDs have been observed in the Fornax cluster, which you can read about in the papers linked to in Jean Tate’s OOTD.

These objects are currently the subject of a lot of debate; are they dwarf galaxies or aren’t they?

Jean Tate summarises this, calling into question if the dwarfs are galaxies or more like globular clusters:

The jury is still well and truly out; however, UCDs do fit several (elliptical) galaxy scaling relationships better than they do globular cluster ones. But perhaps the most intriguing thing is that at least some UCDs seem to have mass-to-light ratios which suggest lots of dark matter, just like dwarf ellipticals (globular clusters seem to have essentially no dark matter) … so perhaps UCDs are not dwarf ellipticals because they are so close to massive cD (giant elliptical) galaxies?

This very interesting paper by Michael Hilker et al includes some very interesting scenarios as to how the UCDs form, including the UCDs being remnants of the centres of galaxies, or the result of  globular clusters merging or that they are indeed dwarf galaxies! The paper also researches whether UCDs have dark matter haloes or not.